1. Field of the Invention
The present invention relates to a process for the preparation of perfluoroalkanes, which have many applications in various fields of industries and are represented by a general formula Rf-F (wherein Rf- is a perfluoroalkyl group represented by F(CF.sub.2).sub.n -- and n is an integer of 2 to 10) and which are exemplified by perfluoroethane (C.sub.2 F.sub.6) for use as a dry etching gas, a process gas in semiconductor production, and the like, perfluorohexane (C.sub.6 F.sub.14) for use as a cleaning agent, a heat medium, and the like from the standpoint of very high stability in chemical and thermal terms, and also for the preparation of iodine pentafluoride (IF.sub.5) useful as a reactive fluorinating agent or a material for intermediate products in the production of fluorine-containing compounds.
2. Description of the Prior Art
It is known to produce C.sub.6 F.sub.14 by effecting a coupling reaction of C.sub.3 F.sub.7 I as a feed material with a metal such as zinc. Disadvantages of this process are, for example, that C.sub.3 F.sub.7 I for use as a feed material is difficult to obtain and that metal iodides are produced as by-products.
In general, methods known as those for producing a perfluoro-compound are as follows:
(a) electrolytic fluorination of a hydrocarbon or a halogenated hydrocarbon; PA1 (b) fluorination of a hydrocarbon with a metal fluoride; PA1 (c) thermal decomposition of a highly fluorinated perfluoro-compound; and PA1 (d) direct fluorination of carbon or a hydrocarbon with gaseous fluorine.
However, the above-mentioned methods are associated with, for example, the following problems.
In the method (a), since various side reactions take place and many by-products other than a target product are produced, separation/purification to obtain the target product is so difficult that the yield of the target product is remarkably reduced.
In the method (b), since many partially fluorinated products are produced, the yield of a perfluoro-compound cannot be increased.
In the method (c), a high temperature is needed for the reaction and the yield of a target product is not good.
In the method (d), there is an advantage that partially fluorinated products are hardly produced. However, gaseous fluorine is so reactive that control of reaction temperature is not easy because of vigorous heat generation involved in the reaction. As a result, C--C bonds are severed and therefore the yield of the target product decreases. In addition, since a risk of explosion and equipment corrosion accompanies the reaction, this method cannot be industrially advantageous.
Alternatively, a method in which a perfluoroalkyl iodide (Rf-I) is reacted with fluorine to obtain a perfluoroalkane and IF.sub.5 is known. According to this method, however, on the one hand, the energy produced at the time of reaction is too large to amounts to about 300 kcal/mol, and on the other hand, the amount of heat being removed is too small when an ordinary heat removing means, for example, an indirect heat exchanger (such as a spiral tube, an outer jacket, and the like) which relies on the removal of sensible heat is used. Accordingly, unless the extent of the reaction itself is controlled, there is a risk that the reaction might become out of control and cause an explosion.
For example, the reaction itself between pentafluoroethyl iodide and gaseous fluorine has been reported by D. E. Johnson et al. (see International Journal of Chemical Kinetics, Vol. 28, 43-55, 1996). In order to utilize this method as an industrial method for producing pentafluoroethane and to obtain the target product in a good yield, the heat of reaction needs to be efficiently removed. However, a concrete technical means to solve the above-mentioned problem has not yet been known.